1. Technical Field
The present invention relates to a discharge lamp lighting device, a control method thereof, and a projector.
2. Related Art
Electric-discharge lamps (hereinafter, referred to as discharge lamp) such as high-pressure mercury lamps or metal halide lamps are used as a light source of projectors. In the discharge lamps, a shape of an electrode is changed by dissipation of the electrode due to the electric discharge and thus flickers in which a discharge start position is suddenly changed may occur. When the flickers occur, the brightness of the discharge lamps is suddenly changed greatly, which is not desirable.
To solve this problem, a discharge lamp driving method is known in which the flickers are suppressed by raising a current value of the AC driving current in a second half of a half cycle of discharge-lamp driving AC current (see JP-T-10-501919).
However, in the driving method disclosed in JP-T-10-501919, the current value is periodically changed. Accordingly, the brightness of the discharge lamp is periodically changed, though the change is smaller than the flickers. For example, when the discharge-lamp driving AC current has 78 Hz, the brightness of the discharge lamp is periodically changed at a frequency of 156 Hz which is double the AC frequency and at a frequency of the integral multiple of the AC frequency.
On the other hand, in a projector employing a liquid crystal panel, since display data is rewritten periodically, the brightness is also periodically changed. For example, when the rewriting frequency is 50 Hz, the brightness is periodically changed at frequencies of 50 Hz and the integral multiple of the rewriting frequency.
When the discharge lamp and the liquid crystal panel are combined, the periodical changes in brightness interfere with each other, thereby causing a scroll noise. For example, in the above-mentioned example, the change in brightness of the discharge lamp of 156 Hz and the change in brightness of the liquid crystal panel of 150 Hz interfere with each other, thereby causing a scroll noise of 6 Hz.
Accordingly, in the past, it was necessary to select the frequency of the discharge-lamp driving AC current so as to be a frequency at which the scroll noise cannot be made to be visible (for example, frequency of the scroll noise is not less than 20 Hz) by the combination with the liquid crystal panel.